8i8 THE POPULAR SCIENCE MONTHLY. 



that while the reception and dissipation of heat by any portion of the 

 earth's surface are equal, the physical properties of ice are such that 

 ice-covered regions lose their heat immediately without being sensi- 

 bly increased in temperature. The effect on temperature of accumu- 

 lations of ice and snow must, therefore, be enormous. " Were it not 

 for the ice," strikingly remarks Dr. Croll, "the summers of Green- 

 land . . . would be as warm as those of England ; but, instead of 

 this, the Greenland summers are colder than our winters. Cover India 

 with an ice-sheet, and its summers would be colder than those of Eng- 

 land." Even on our own coasts the grounding of a single berg ap- 

 preciably lowers the temperature and greatly increases the danger of 

 frosts. 



On the hemisphere whose sum,mers occurred in aphelion during 

 the period of high eccentricity, an exactly opposite tendency would 

 be manifested : the snow and ice would gradually melt and perhaps 

 entirely disappear, and vegetation might flourish even under the pole. 

 That hemisphere would then enjoy an interglacial period. These 

 periods occupy an important place in the theory under examination. 



Dr. Croll then proceeds to show how the accumulation of ice in 

 polar regions would affect the general oceanic circulation : " Owing to 

 the difference between the temperature of the equator and the poles 

 there is a constant flow of air from the poles to the equator. It is to 

 this that the trade-winds owe their existence. Now, as the strength 

 of these winds, as a general rule, will depend on the difference of tem- 

 perature that may exist between the equator and higher latitudes, it 

 follows -that the trades on the cold hemisphere will be stronger than 

 those on the warm. . . . Suppose, now, the northern hemisphere to be 

 the cold one. The northeast trade-winds of this hemisphere will far 

 exceed in strength the southeast trade-winds of the southern hemi- 

 sphere. The median-line between the trades will consequently lie to 

 a very considerable distance to the south of the equator. . . . Let us 

 now consider how this would affect the Gulf Stream. The South 

 American Continent is shaped somewhat in the form of a triangle, with 

 one of its angular corners, called Cape St. Roque, pointing eastward. 

 The equatorial current of the Atlantic impinges against this corner, 

 but, as the greater portion of the current lies a little to the north of 

 the corner, it flows westward into the Gulf of Mexico and forms the 

 Gulf Stream. . . . Now, it is perfectly obvious that the shifting of the 

 equatorial current of the Atlantic only a few degrees to the south of 

 its present position — a thing which would certainly take j^lace under 

 the conditions we have been detailing — would turn the entire cur- 

 rent " to the south of Cape St. Roque, and thence along the Brazilian 

 shores and into the Southern Ocean, and " the Gulf Stream would 

 consequently be stopped." 



Now, it is quite manifest that if the wind theory of oceanic circula- 

 tion is incorrect — and, as ah-eady shown, its correctness has not been 



